1. Technical Field
This disclosure relates to switching power supplies, including buck-boost converters.
2. Description of Related Art
Buck-boost converters may be used to produce a constant DC output voltage, notwithstanding a DC input voltage which may be higher, lower, or equal to the output voltage. They may operate in a buck mode, a boost mode, and/or a buck-boost mode. The converter is considered to be in a buck mode of operation when the input voltage is higher than the output voltage, in a boost mode of operation when the input voltage is lower than the output voltage, and in a buck-boost mode of operation when the input voltage is approximately the same as the output voltage.
Buck-boost converters may include an inductance having a first and a second connection. They may also include four electronic switches: an electronic switch A configured to cause the first connection to the inductance to be controllably coupled to an input voltage source; an electronic switch B configured to cause the first connection to the inductance to be controllably coupled to a ground; an electronic switch C configured to cause the second connection to the inductance to be controllably coupled to a ground; and an electronic switch D configured to cause the second connection to the inductance to be controllably coupled to a load output. They may also include a capacitance configured to filter the load output. They may also include a switch controller configured to cause switches A, B, C, & D to operate in a manner which controllably causes the converter to operate in the buck, boost, and/or buck-boost mode of operation:
Buck-boost converters may utilize two phase AC-BD switching with a voltage mode control loop to set the duty cycle (i.e. the point at which the AC phase is switched to the BD phase). This approach may produce almost any desired step-up or step-down voltage ratio. However the inductor current that results may be much larger than it needs to be to support any given load. This may result in poor efficiency, since resistive losses may increase dramatically and the deliverable output current (for a given switch current limitation) may be greatly reduced.
Buck-boost converters may cause switches A and C to close during one interval, switches A and D to close during another interval, and switches B and D to close during a still further interval. This approach may use a dual, pulse-width modulated (“PWM”) ramp as a means of transitioning between the boost, buck-boost, and buck modes of operation. However, this approach may result in perturbations to the operating state when transitioning between these modes of operation.